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Northeastern Illinois Air Photo Archive and Webmap

Introduction

CMAP maintains a large and comprehensive collection of aerial photographs for the northeastern Illinois region dating from 1970 through 2001. The archive numbers some 6,300 images that cover six counties. The photographs are black and white half-tone enlargements printed on a translucent mylar base measuring 34" x 34." Each image covers approximately four square miles, arranged by Public Land Survey sections within townships.

In 2013 CMAP began scanning this collection to make it more accessible to the public through a free interactive web mapping application. Converting these images to a digital format was determined to be the best alternative for archiving the aerial photographs and enhancing user access without further degradation to the original images. This is an ongoing project that will take several years to complete.

Full-frame image including part of Fermilab.

Historic aerial photographs are valuable resources for planning activities including: landscape and land use analysis, assessment of environmental impacts, development projects, and education. The images provide a method for examining changes in properties, neighborhoods and land use in general. In addition, historical, environmental, or architectural information about particular sites over a period of time can also be examined.

The CMAP Imagery Explorer

Scanned images are available for preview and download on our Imagery Explorer site. You can use the online map to navigate to your area of interest, and use the drawing tools to outline the area that you want images for. The blue boxes that appear in the Results step represent individual scanned images; you can open a small preview window and download the image and associated files.

These images are scans of the complete mylar sheets, including collar information such as: date of flight, township/range/section numbers, and scale bar. They are in TIFF format, with accompanying .xml and "world" (.tfwx) files that allow the images to display in a GIS package such as ArcGIS. Most images are between 70 and 100 megabytes.

Limitations of the Imagery

While the images are georeferenced, they are not orthorectified. Orthorectification is a process that incorporates elevation and image sensor data to correct for distortion in the image caused by irregular terrain, camera tilt, and other factors. The georeferencing process described elsewhere on this page gets the imagery "in the ballpark," and is best suited for the qualitative assessment of historical land use patterns at a scale of 1:10,000 or higher. While there has been no formal assessment of spatial accuracy, variances of 50' or greater have been observed in some locations when compared with orthorectified imagery.

These images are scans of half-tone enlargements, and not of the original (continuous-tone) prints nor of the original films. An unavoidable byproduct of using a digital scanner on half-tone prints is a patterning known as moiré that is visible at certain scales. While the resultant image is considered unacceptable for engineering, remote-sensing, or other technical applications, this is the best available in the absence of the original photographs. More information on the technical aspects of this project can be found below.

Enlargement to 1:4,800. Block in the center is the future site of the Willis Tower.

Moiré patterning evident in Chicago River (lower left).

Frequently-Asked Questions

What are the numbers on the top of the photos?

The T and R numbers refer to Township and Range, while the numbers in the four boxes are Section numbers; all are part of the Public Land Survey System (PLSS). More information about the PLSS can be found here.

How can I measure the scale of the air photo I've downloaded?

There is a scale bar at the bottom of each image.

What scale are the photos?

The full-size mylar prints are 1" = 400', or 1:4,800. Printing or viewing on-screen may cause the imagery to display at a different scale. Always include the scale bar when printing an image.

Which direction is North on the photos?

The top. There is a north arrow along the bottom of the image next to the scale bar.

Can I get latitude and longitude or GPS coordinates from the photos?

The imagery includes an embedded "world file" which will provide approximate coordinates when viewed with GIS software.

Why are only some aerial photographs online?

Existing resources allow us to scan one year's worth of imagery per year; we hope to have the entire set available by 2019. Due to heavy usage of these photos over the decades, some may have gone missing or were damaged; in those instances a "Missing Image" icon will appear when selecting it.

Where are the photos for Kendall County?

Kendall County was not a part of the NIPC planning area, so you will not find any imagery for Kendall in most cases. An exception is the 1970 set, where images are available for Bristol, Na-Au-Say, and Oswego Townships.

Do you have any photos earlier than the ones you list?

No. The Northeastern Illinois Planning Commission flew portions of the region in other years; contact prints of those flights were transferred to the UIC Map Library.

Technical Information

History

The Northeastern Illinois Planning Commission contracted for aerial photography every five years beginning in 1970 to monitor the distribution and change of land use in the region. The mylar enlargements were available for inspection at NIPC offices, and Diazo "blackline" reproductions could be purchased. As the Diazo printing process gave way to high-speed digital scanning, the quality of the reproductions suffered. While there is still a great deal of interest in the images, there was no longer a way to provide a cost-effective means of reproduction that yielded a useable image.

This scanning effort, then, serves two purposes: first, to create a digital archive of a unique historical resource; and second, to make the scans freely available for download through a publicly-accessible interface. While not without flaws, these scans represent the best available solution for extending the utility of the photographs.

Scanning Process

All scanning takes place at CMAP's offices, using a Vidar HD3630 wide-format scanner. While the scanner has the capability of 600 DPI resolution, testing at CMAP determined that the optimal resolution was between 250 and 300 DPI; since the images on the mylars are not continuous-tone, scanning at a higher resolution yielded no improvement in quality. Descreening was used to help minimize the moiré effects. After repeated tests, these settings were also established:

Image type: Grayscale (8 bit)

Levels:

Contrast: 0

Brightness: 0

Gamma: 2.0

White point: 255

Black point: 0

Sharpen/Smoothen:

Intensity: 0

Radius: 1.0

Threshold: 0

Smoothen: 0

The scanning process was tracked using an Access database, with a separate table for each year and each table containing one record for each potential scan ("potential" because some images have been lost or damaged over the years). The records were pre-populated with as much meta-data as possible (township/range/section numbers, political township, contractor name); during scanning, the technician added additional information such as the date of the photograph, and verified the township name.

A Note About the 1970 Set: Many of the mylar sheets that were scanned to create the 1970 images had been subjected to considerable wear over the decades that they were in active use. This wear resulted in many of the original scanned images being very dark, some to the point of being nearly unusable; frequently there were also noticeable differences in darkness between photos of adjacent areas. To increase the amount of detail visible in the photos – as well as to improve their general aesthetic – a pair of "droplets" were created in Adobe Photoshop. These allowed pre-defined sets of adjustments to be applied to large batches of images simultaneously. One droplet was used on the darkest 25% of the 1970 images, which significantly boosted their contrast and brightness; the other, which modified the images to a much smaller extent, was applied to the remaining 75% of photos. The result is a set of images that is much more legible and more consistent, photo-to-photo.

Georeferencing

Each scanned image was georeferenced with Esri's ArcMap software package. The primary data source used to assign control points to the scanned images was orthorectified aerial imagery from 2005. Supplemental data sources included orthoimagery from 2010 (with higher resolution than the 2005 imagery), Public Land Survey System boundaries, and major road centerlines from 2005. Wherever possible, the corners of short buildings that had certainly not changed in the years since being photographed were used as control points.

A minimum of six control points were created for each image, although 12-15 were desirable. These points were distributed around the image as evenly as possible in an attempt to minimize distortion. With very few exceptions, only 1st order polynomial (affine) transformations were used to georeference the images; while higher-order polynomial transformations are good for correcting distortions, these images (being scanned from mylar sheets that don't warp with age) are unlikely to contain significant distortions, and the transformations would likely introduce distortion into areas of the image without many control points.